Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why do photons have momentum yet no mass?

  1. Mar 7, 2013 #1
    I imagine this gets asked a lot but i'm looking for an in depth explanation since none of the others I've found are detailed enough.

    I understand that p = mv is not applicable near the speed of light and I think (but am unsure) that the correct equation to use is e = pc.

    Assuming that e = pc is the correct equation to use then what does 'p' represent?

    Assuming e = pc is the wrong equation then what is the right one?

    And can someone (if you can be bothered to waste time on a dumbass like me) explain why p = mv is not applicable?

  2. jcsd
  3. Mar 7, 2013 #2
    No one knows 'why' a photon exists nor why it has the characteristics it does....
    the fundamental particles of this universe, as well as mass, energy, time, distance,etc....were all 'unified' [combined] in one entity very early in our universe....that was a very high energy unstable environment which underwent what is called 'spontaneous symmetry breaking'....meaning the original entity broke down to a lower energy state and became all those different entities I listed...... which is what we observe today.

    with p = mv, what would you use the the 'm' of a photon??

    For a correct description, see here:


    Unlike a matter particle, a photon always moves in a vacuum at 'c' and if a photon has higher energy it has higher frequency....so a gamma ray, for example, is very energetic [high frequency] visible light less so.
  4. Mar 7, 2013 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Please note that even in the classical treatment of light as EM wave and not photons, there is still a momentum associated with light! This is a standard treatment of classical E&M. So one does not need to have a picture of massless photons to already realize that light can have a momentum.

  5. Mar 7, 2013 #4


    User Avatar

    Staff: Mentor

    The relativistic relationship between a particle's energy, momentum and mass is

    $$E^2 = (pc)^2 + (mc^2)^2$$

    where m is what many people call "rest mass" but physicists usually call just "mass." Setting m = 0 (as for a photon) gives you E = pc.
  6. Mar 7, 2013 #5
    It IS applicable NEAR but not AT the speed of light....

    edit: sloppy explanation by me...see last line of the next post..by Bill_K..
    Last edited: Mar 7, 2013
  7. Mar 7, 2013 #6


    User Avatar
    Science Advisor

    Only if you mean m to be the antiquated concept of relativistic mass. In terms of the usual rest mass, p = γmv.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook